As he fights them we see him punching his side where the wound is and there's blood on the snow.

What the heck is he doing? Is this some kind of dark side healing method?

EDIT:
There is an indentical question of this at SciFi SE which was posted about an hour before mine. Neither of them has an accepted response but there are some good theories. I'd like to see someone come up with a theory that can be backed up though.

My thought when I saw the movie was that he had lost feeling in the injured area and was trying to smack it to get some feeling back.
– forrestbroDec 20 '15 at 16:49

My theory - mind, I have nothing to back this up except Ren's characterization in the film - is that he's actually trying to injure himself to the point he needs life support, to make himself more like Vader, in the hopes that that will increase his powers.
– littlebeastJan 5 '16 at 1:39

2

I've done this to areas of sharp pain before to create a duller pain to mask how much it initially hurts. If the bowcaster caused a sharp pain, he could be trying to distract himself from that.
– Broots WaymbJan 5 '16 at 19:00

8 Answers
8

I won't repost my answer completely, but to summarise, an alternate, much simpler theory based entirely on things that exist in real life is that he's trying to beat away the pain from the wound that Chewie gave him earlier: turning a distracting, acute, sharp pain that kicks in when he makes certain movements into a blunter, persistent, worse but more predictable pain that is easier to tough out and ignore.

This is something I've done in real life when sharp (non-wound...) pains like cramp or muscle strain momentarily prevent me from doing important things. I wouldn't recommend it... But it feels like it helps at the time, for a few seconds at least. I'm sure I've seen lightly-injured sports players do it

Basically, pain is less distracting when it's predictable or persistent.

Of course, this is not a sensible thing to do, because it makes the actual wound worse, and the numb feeling wears off in seconds - but we shouldn't expect Kylo to be behaving sensibly at this point.

...if a muscle seizes up or starts spasming (eg: with Charlie Horse [cramp]) smacking it a bit can sometimes help it to unclench and start obeying orders properly

Based on the fact that it coincides with other visual reminders of his injury (blood on the snow, limping, roaring in frustration) I also suspect that, from a storytelling point of view, this is intended to remind the viewer that he's fighting at far below his usual ability.

LOTS of people question why Finn's fight and Rey's fight with Kylo aren't much more one-sided, and react as if an injured character showing actual signs of injury is some kind of plot hole (I blame Dark Knight Rises for this...). I suspect Kylo's very noticeable behaviours were a (doomed) attempt by the film-makers to remind viewers that normally, serious injuries are supposed to actually impede a character's fighting abilities, and not just be forgotten about the moment the camera angle changes...

The dark warrior also possessed a strong tolerance to physical pain, able to fight even after being shot by Chewbacca and wounded by Finn and Rey. Kylo could be seen beating his wound to utilize his pain and tap into the dark side of the force.

This seems like circular attribution. Wookiepedia writer made up that explanation to explain what they saw in the film (it's not cited from canon), and now it's being cited as if it is a valid explanation from canon itself.
– DVKDec 24 '15 at 6:36

This line appears to have since been removed from that site, there's now no mention of the wound-beating.
– user56reinstatemonica8Jan 18 '16 at 10:44

I've unchecked this as the accepted answer due to the reasons stated in other comments. The Wookpedia source does appear to have been redacted.
– sanpacoMar 30 '16 at 21:40

I think it's more based in his character than anything else. Kylo isn't just ashamed of his weakness, he passionately despises it. It's an externalization of his inadequacies throughout the movie. In this moment, we're seeing him explode, not only lashing out at Finn and Rey, but at his grief, embarrassment, and even physical pain. I'm not sure how clear an answer this is, but just from watching the movie alone, I tried to consider it purely from a character and thematic point of view.

To be honest, and this is after reading most of the comments here and drawing my own conclusion, I feel like a mix of things is my answer;
Dark Force/Force Rage is my main. If you watch when he actually gets hit with it, his reaction is to then stand up and walk away; I feel like that moment is underrated, for him. The sheer strength that would take is immense- it would be a good deal easier if he was fuelled by dark force at that present moment in time? And the way he takes a moment to look up at the reactions of Rey and Finn, and he looks angry too, makes me think that the Dark Force theory is quite likely; the fact that you become especially drained after Dark Force wears off would also explain why towards the end he staggers around like a drunkard and can barely defend himself, he's definitely on the defensive more so than you'd expect him to be. My other reasoning is, when Rey says 'the force' and seems to clear her mind, becoming calm, Kylo seems to be the complete opposite. Rey seems to be using the light to fuel her, as after this point she basically just kicks his arse, and Kylo is being fuelled by the dark, which takes much more out of you. Combine this with the freezing cold, and the fact he's now been fighting for quite a while with a serious injury, and it makes sense that Rey would suddenly be able to beat him. He seems to be lagging a little before each time he does it, deliberately hurting himself more, to keep fuelling the Dark Force. Another reason could be the more you use the Dark Force/Force Rage, the less affected you are by the light/the darker you become and as we know, the light is plaguing him to the extent he thought -the scene earlier- could stop it (though whether this works or not remains to be seen)- his primary goals are to vanquish the light inside of him and become as powerful, if not more powerful than Darth Vader. I feel like using Dark Force like this would definitely help make this happen.

Although, I don't think Dark Force is the only reason. I think it's also to help remind viewers how important it is that he was injured, and that he's not fighting at full strength (or he'd have easily beaten Rey). And to foreshadow this injury will cost him later on. (Hence, he loses the fight)
Another reason is indeed the type of person he seems to be- he lashes out at everyone and everything whilst angry and frustrated, I think in this fight its apparent he is both. The wound, since it would be causing him pain, is likely what he chose to focus on, hitting himself to get the anger out, and also because he hates the fact he's even wounded, almost punishing himself for allowing it to happen.
It could also be a scare tactic towards Rey and Finn, they seem shocked and intimidated the first time we see him do it.
And lastly, it could indeed be a medical reason, to numb, or change the type of pain, which I have done before now too- personally, I find it's also somewhat instinctive- or to stop a muscle spasm or unresponsive/seized up muscles.
Most of this is just repeating what others said, admittedly, but here's my full take haha.

I don't think it's any kung fu or dark side act. After losing blood, he's weak entering the forest. He's staggering and probably getting tunnel vision from blood loss. I believe he's hitting himself to focus, trying to get adrenaline and endorphins pumping to help himself fight. Like a runner's high, his pain would be dulled enough to get the job done.

I have a few theories based on Kylo's characterization, but they are just that - theories.

One is that Kylo Ren is more upset and grief-stricken over the scene immediately preceding than he lets on. In the novelization and in other concept artwork and images, Kylo Ren is shown to be more conflicted than he appears in the final film. Him causing pain to himself could be a way of subconsciously "punishing himself" for the actions he took in that scene.

Another explanation is the one offered by Wookiepedia - he is attempting to tap into the Dark Side by harnessing his pain. One of the main themes of the Dark Side throughout the films has been "focus". Palatine mentions this to Anakin as early as Episode III when he feels Anakin beginning to embrace the Dark Side - he mentions that Anakin's hatred "...gives you focus." Granted Kylo Ren wasn't privy to these conversations, but his training with Supreme Leader Snoke could have led him to believe that pain, anger, and rage lead to increased Dark Side power, hence his inflicting more pain on himself in a critical moment when being focused and present is key.

A final explanation is that it was more like a wounded soldier trying to beat away pain, or distract himself from the pain by beating on it in the manner he does. One interesting thing about the blaster is that it seems to have some physical component - twice in the film we are shown someone being shot by a blaster and bleeding, which seems to suggest that blasters are not "laser guns" but in fact are hyper-acellerating some sort of projectile. Given the fact that every other time Chewbacca or Han shoot someone with the Bowcaster, they're shown flying off their feet from the impact, and Kylo Ren does not, it seems to suggest that perhaps whatever projectile the Bowcaster fires is still lodged inside of Kylo Ren, leading to him acting out in the way he does.

Bear in mind this is all speculation based on evidence given in canonical sources only.

I honestly think that there was nothing special about this. I think they were just trying to make some sort of passive justification of why he wouldn't just force grip his opponents and take control of the situation. I mean, he could stop a blaster but had to saber fight Rey and Finn? No. I think this was a plot device to show that he was injured in such a way that was preventing him for using the telekinetic aspect of the force which is also why he was having hard time grabbing Luke's saber and then Rey was able to over him. He couldn't force push or pull or anything during the fight; he left with nothing but his manual skills. There may end up being some additional explanation to this in the coming movie.

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This a Kung-Fu move that stop internal bleeding from occurring especially since it was a blaster/gun shot. The wound that Chewie gave him was located near some minor organs so he was really just trying to heal it, although don't try it, it hurts like shit.